To cope with challenges of a wireless broadband technology and keep a leading advantage of the 3GPP the 3rd Generation Partnership Project) network, the 3GPP formulates an LTE (Long Term Evolution) plan. A new mobile communications network architecture is defined under the guidance of the evolution plan. The architecture retains only a packet domain, and therefore may be referred to as an EPS (evolved packet system).
A core network of the EPS mainly includes three logical functional entities: an MME (mobility management entity), an S-GW (serving gateway), a P-GW (packet data network gateway), and the three functional entities are core network devices of the EPS network. The MME is responsible for functions such as NAS (non-access stratum) signaling processing, security, and mobility management, and corresponds to a control plane part of an SGSN (Serving GPRS Support Node, serving GPRS (general packet radio service) support node) in a current UMTS (universal mobile telecommunications system); the S-GW is responsible for a local mobility anchor and a mobility anchor in the 3GPP system and for lawful interception of related information; and the P-GW is responsible for related functions such as policy enforcement and charging and lawful interception.
In an existing EPC (evolved packet core, where the EPC is a part of the EPS), a default bearer is established when UE initiates attach and is retained all the time when a PDN (packet data network) connection exists, to provide an IP connection for access of the UE (user equipment) to a PDN. However, in some application scenarios, data exchange does not need to be performed by using an IP connection, for example, for some services of MTC (machine type communication), interaction may be performed in a form of an SMS (short message service) or another service. Therefore, for user equipment having some specific service features, temporarily, no default bearer is established during attachment of the user equipment, some services may be completed in a non-IP connection manner. If data transfer needs to be performed by using a user bearer (or referred to as a data bearer), a bearer establishment process is further initiated.
When there is no default bearer, small data may be transferred by using signaling between an MTC-IWF (MTC interworking function) and an SGSN/MME. However, when UE is handed over between cells, in an existing handover process in an EPC, a data packet needs to be forwarded from a source eNB to a target eNB, and if direct forwarding between the eNBs is not supported, the source eNB indirectly forwards the data packet to the target eNB by using a source S-GW and a target S-GW, which requires data bearers to be established between the eNBs and the S-GWs. However, actually, when small data is transmitted in an existing EPC, the small data is transmitted by using signal connections between an MME, an eNB, and UE, and no data bearer exists between the eNB and an S-GW. Therefore, the existing handover process in the EPC is not applicable to a handover process of a network without a default bearer.